Embodiments of the invention relate generally to buoyancy cans for tensioning risers. More particularly, embodiments of the invention relate to a tube buoyancy can system for providing an adjustable tension load to a top-tensioned riser.
Marine risers are typically employed for offshore platforms to provide conduits between the platform and the seabed. Marine drilling risers are used to guide a drillstring and convey fluids used during various offshore drilling operations. Marine production risers establish a flow path for hydrocarbons produced from a subsea reservoir to a production facility located at the water surface. Other types of marine risers exist. Even so, the functions of marine risers can be generally summarized as the transfer of matter, power or signals between the seabed and the water surface.
Common to all types of marine risers is that due to their weight, a certain amount of vertical force is necessary to keep the riser upright and prevent it from dropping to the seafloor. Moreover, vertically arranged marine risers must be over-tensioned beyond their self weight in order to limit the deflections and stresses in the riser due to exposure to the dynamic ocean environment. Such vertically arranged and tensioned risers are commonly known as top tension risers. In addition to the tension requirement, risers attached to a floating drilling or production vessel must be decoupled from the vessel's heave motion, which is induced by wave action.
The two commonly used types of riser tensioning devices are hydraulic actuators and buoyancy cans. For a hydraulic riser tensioner, hydraulic actuators are attached between the vessel and the top of the riser. Vessel heave is compensated by actuator stroke, while the riser tension is maintained at a substantially constant level by actively controlling the hydraulic pressure. Buoyancy can tensioners, on the other hand, are passive devices attached to the upper portion of risers below the waterline. The riser tension is provided by buoyancy, while vessel heave is compensated by allowing the buoyancy can to slide up and down relative to the host vessel in sleeve-type guides. Conventionally, both hydraulic tensioners and buoyancy cans are applied to a single riser. Where a plurality of risers is to be supported, each riser is tensioned individually by a separate tensioner.
Irrespective of the type of riser tensioner, the functional requirements for operation in deep water and harsh ocean environments provide significant technological challenges for their design. Riser weight and consequently the tensioner capacity requirement increase with water depth. Tensioner stroke requirements increase with increasing motions of the host vessel, which, in turn, are a result of the severity of the wave environment. Some buoyancy cans, such as those disclosed by U.S. Pat. No. 6,884,003, allow the support of multiple risers. When such multi-riser buoyancy cans operate with less than the full complement of risers, the buoyancy can must be ballasted to prevent over-tensioning the risers. Due to the additional ballast, heave periods of the buoyancy cans may shift into a range where appreciable wave energy exists, resulting in increased dynamic loads to the risers. Because of these design constraints, the tensioners used for the latest generation of drilling or production vessels are large, complex, and expensive. For some applications, the load and stroke requirements have reached the limits of existing tensioner technology.
Exploration and production in even deeper waters and harsher environments demand new technologies that overcome current limitations. Moreover, operational flexibility and cost reduction on marine riser systems has become increasingly important for the oil & gas industry, as this industry is confronted with more economically challenging reservoirs in deep waters. Accordingly, embodiments of the invention are directed to buoyancy can systems and associated methods that seek to overcome these and other limitations of the prior art.